Ensure a graceful shutdown of the containers

[ro-tex]

Docker is already sending the right signal to siad and it's starting the graceful shutdown. The issue is that shutting down the renter is quite slow sometimes and docker is not patient enough for it. We need to ensure it waits for the full shutdown and doesn't trigger any timeouts that interrupt it. On the other hand we need to be vigilant to not allow a crash to block the container from ever shutting down.